Need help to spec wood cross beams on wood sailboat cradle

Discussion in 'Sailboats' started by Jay and Ebben, Sep 29, 2012.

  1. Jay and Ebben
    Joined: Jan 2011
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    Location: Vermont - U.S.A.

    Jay and Ebben BilgeRat

    I have not had much luck understanding the basic formulas to spec out the number and sizes of cross beams I need to support a 33,000 lbs sailboat. I will be jacking up this hull in about 10 days to build a cradle underneath and then I intend on rolling it (new cradle and all) on 10" dia. wood rollers (3' sections of used phone poles) up to its new shelter for a full rebuild.

    The cradle will be 8' wide. The length still to be determined.

    All wood is North Eastern White Pine.
    Available for 8' cross members are;
    2 - 10" x 10"
    4 6" x 6"
    2 6" x 9"

    These cross beams will be supported on their outer ends by 6" x 10" which of course will be running for/aft.

    Thank you for your help.

    You can try this link to my new blog to catch up on the project and see the hull. I am not sure whether the link works yet.... never posted it before!

    http://voyageoffshore.blogspot.com/

    Thanks!
     
  2. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    You are using wood to build a cradle, after building that steel shed ?? The quality of timber is never easy to assess on the spot, let alone over the internet.

    Don't go mucking around with 15 tonne of steel boat on wooden rollers. They will scoot around, bury themselves in the mud, and someone will get their fingers crushed pushing them into position.

    get some steel section, weld up a cradle. If you cant scavenge some trucks axles and wheels, mount the cradle on top of lengths of timber to use as sled
     
  3. Jay and Ebben
    Joined: Jan 2011
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    Location: Vermont - U.S.A.

    Jay and Ebben BilgeRat

    I agree metal would be better and I have looked for months for axles and such to be modified with no luck in the search. The distance I need to move the boat is about 120 yards which is doable. I have a dozer and two heavy tractors (one with a ten ton towing winch which pulls very slow with good control). The main reason I have taken so long to find a boat is finances are tight. I do have a stack of dry fresh beams to pull from as needed. Call it "mucking around" but that is what I have to do. Moitessier went to sea with phone pole masts when he would rather have had other materials at hand. I clearly am no Moitessier, but I have to follow his practicality of method.

    I will keep my eyes open still for steel beams and such. But for now my initial question still stands.

    Thanks!
     
  4. rwatson
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    Location: Tasmania,Australia

    rwatson Senior Member

    Ok, well if you are sure thats the only approach, I would still abandon wooden rollers, and plan to slide the thing with your heavy equipment. if you keep the timber wet, it will slide ok. It works with heavy equipment like dozers.

    My approach would be allow the worst case of 1/2 the boats weight for the side loadings. If the whole thing leans to one side suddenly, there is good support with a safety allowance, as most of the weight would be on the base of the platform. You are basically just building a launching cradle that wont go near water.

    Maybe you can use local building beam ratings. See how much the timber you have will hold. You can get free strength (loading) tables from the local hardware shop hoping to sell you the timber, or a friendly draftsman with beam tables. Dont forget, bolting two or more beams together increases the strength greatly.

    If you are ultra conservative, and perhaps insurance is a potential problem, you should probably get a local engineer to design the scaffolding. It would be no more difficult than calculating a large wall loading, and not very expensive.

    Moitessier was involved in several shipwrecks - you really dont need any kind of drama at the start of this big project.
     
  5. Jay and Ebben
    Joined: Jan 2011
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    Jay and Ebben BilgeRat

    Please help with this wood beam spec table?

    I can still use some guidance here please.

    www.awc.org/pdf/wsdd/c2b.pdf

    From the link above, page 69 refers to an 8 foot span (my cradle width) and a 10" x 10" beam. Tables such as this all seem to refer only to "even load distribution". My cradle would clearly have center loaded cross beams.

    For starters, is it more or less correct to see that one 10 x 10 can support somewhere between 10717 lbs to 23815 lbs? of evenly distributed load?

    I am not clear as to how to apply the bending stress (Fb) factor.

    Can anyone help me understand how to narrow this further?

    To keep it simple I have two 10" x 10"s and up to eight 6" x 10"s of eastern White Pine to use for the cross bedders.

    Again, my boat weighs about 33,000 lbs

    Thank you!
     
  6. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    I'm your having no luck with your links. My first thoughts are to have you take Watsons advice. Do not use rollers on an earthen base. If you do use rollers then it will be necessary to use a lot of them. There is the matter of the area of the roller contact patch and the soil bearing capacity.

    You can not say that a 10 x 10 will support X number of pounds. Load capacity involves too many variables. A structural member can be loaded as a beam or a column. In beam load situations, the AISC manual lists 31 different ways that a beam may be loaded. The capacity varies considerably with the manner in which the load is applied. One of the most signifigant factors is the distance between the supporting elements. The formulae treats distance, usually notated as small letter l, exponentialaly at the third or fourth power when calculating deflection. The exponent depends on the manner of loading. This stuff is way too sticky to mess with unless you plan on a structural engineering career.

    I do not wish to discourage your efforts, you can do this. I am a believer in good old yankee ingenuity, however, the safety of you, your helpers, and your boat are too important to treat lightly.

    I urge you to consider the wisdom of consulting a structural engineer, a crusty old boat yard guy who has experience with big boat hauling, or perhaps a professional rigger. This ain't a dinghy you are figuring to move.
     
  7. Jay and Ebben
    Joined: Jan 2011
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    Jay and Ebben BilgeRat

    I would think that spending my time and money to obtain an engineering degree to solve this problem would defeat the practicality of my entire project at its infancy and entering the question of columns to the equation is a concern that I don't see being an issue. This being a keel boat, the beam is center loaded. The wood is dry and of good quality, with minimal, tight knots for the species of White Pine.

    I have off-cuts from approximately 20 poles which I will cut to about 3' lengths - they are all about 10" dia. and I will match up by size. The ground will be dry and hard on moving day. If there are problems I can remove the rollers but simply dragging it would seem to me to incur unimaginable friction, and when starting and stopping each pull the rocking forces would be alarming?

    Yes there are concerns here but I am trying to recognize them and build in a safety margin that makes it a safe move. Don't get me wrong here, I would LOVE to hire an engineer but if I had that type of budget I would hire out much of the other work or perhaps even purchase a less labor demanding vessel...

    ...with all respect and a poke-in the-ribs.

    Right now I think a minimum is:
    two 10 x 10 cross braces
    three 6 x 10 cross braces

    Which when pulled from the low end of the chart above should yield over 40,000 lbs of support (albeit for an evenly distributed load). An extra 6 X 10 would allow for another 6200 lbs.

    I remain hopeful a cradle genius will see this thread eventually and throw me a bone of experienced advice.

    Sail on!
     
  8. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I completely agree that you don't need an engineering degree for this. I also disagree in that wooden rollers aren't a good choice.

    Wooden rollers will need to go over a plywood base. This simply means enough sheets of 3/4" to cover the ground under the cradle and a bit in front of the cradle and you move from back to front during the pull, to save the need for a bunch of sheets.

    Wooden rollers are soft and will absorb, deflect and bend to changes in cradle load, shape and earth contact. This is fair better then a bent metal roller or a stuck wheel in a rut. I do agree in the need for non-earth contact, but this is what the plywood is for, each a 32 square foot contact spreader.

    I think your current scantlings are sufficient, though tossing another 6x10 in there wouldn't hurt. The key will be a good smooth roll, so the loads are evenly distributed, so focus on the run out path and making a path that will permit an upright move.
     
  9. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Go back to square one. What makes you think your little dozer and friends will be able to shift it? You need a decent haul road stabilized with ABC and about five axles to provide flotation. Or three dually axles. Got a drawbar pull rating on that dozer?

    rent a hydraulic sailboat trailer for the day.

    This might do, I'd try to find a three axle dually. You'll need at least an 8" lift of ABC, but it is cheap.

    http://www.marinasandtransport.com/...=class_search.pl?property0=Hydraulic Trailers
     
  10. philSweet
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    philSweet Senior Member

    My concern with rollers are that I don't see any equipment on hand that can deal with a fouled roller. Also it looks like it needs to go uphill and make a 90 degree turn. It will roll a lot easier downhill than up. Par's plywood idea would work if the roadbase was properly prepared. I think you might end up going through a pallet of 3/4 cdx though. If the frame comes off a 12" roller thats a lot of tipping.

    If you are determined to do this, prepare a ten foot section and try to drag it those ten feet. That will tell you more than anything else.

    You're going to have to get the bloody thing back out again when you are done, so you might as well figure the cost of doing it twice. That makes the road look better.
     
  11. PAR
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    PAR Yacht Designer/Builder

    A skid or sled will work too, but I don't think you need to over engineer this thing, just make it stout enough to get the job done. I also don't think you'll need all that many sheets of plywood, though the fewer you have, the more you'll have to shift from back to front. The roadway's condition is quite important, but in your area the soil is dense and hard, so a simple grading with some shovels or a loader will do.
     

  12. kvsgkvng
    Joined: Jan 2012
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    kvsgkvng Senior Member

    Please contact me off list. I will be able to help you as a licensed structural engineer. Regards.
     
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